Method and apparatus for manufacturing tubes by rolling

ABSTRACT

A method for manufacturing a tube made of a non-ferrous material, particularly a tube made of mainly copper, by rolling, in which method, in the first working step the tube billet is worked by rolling with conical rolls, so that mainly owing to the deformation resistance, the temperature of the billet under operation rises up to the recrystallization range, at least on the spot that is being worked. Essentially immediately after the first working step, the tube billet is subjected to at least one second working step with a second set of conical rolls, in which case the tube billet is maintained, at least during the first working step and at least a second working step in non-oxidizing conditions. The invention also relates to an apparatus.

[0001] The present invention relates to a method according to the preamble of claim 1 for manufacturing tubes by rolling. The invention also relates to an apparatus according to claim 15.

[0002] From the U.S. Pat. No. 4,876,870, there is known a method for manufacturing tubes of a non-ferrous metal, where a continuously cast billet is cold worked for instance by planetary rolling, so that owing to the influence of deformation resistance, the temperature of the worked material rises to the recrystallization range. In said publication, cold working generally means a process where the temperature of the billet under operation is normal when starting the working, but rises along with the process essentially higher than in an ordinary cold working operation, i.e. up to the recrystallization range of the material. A planetary rolling arrangement is disclosed in the U.S. Pat. No. 3,735,617, where three conical rolls are arranged at angles of 120° with respect to each other. The rolls rotate both around their own axis and around the center of the planetary housing. In said arrangement, the mainly conically narrowing shape of the rolls is essentially narrowed in the proceeding direction of the material to be rolled. There are also known corresponding arrangements where the rolls are arranged in a reversed position with respect to the proceeding direction of the rolled material, in which case their conical shape is narrowed against the proceeding direction of the material to be rolled. The U.S. Pat. No. 4,510,787 introduces a method for manufacturing hollow rods, where one possibility is to employ mainly conical rolls that are narrowed in an opposite direction than the proceeding direction of the rolled material. From the GB application 2019281 A, there also is known a planetary mill where the axes of the rolls are parallel with the proceeding direction of the tube billet to be rolled. Yet another arrangement known in the prior art is illustrated in FIG. 1.

[0003] Copper tubes have been manufactured extremely successfully by using the method of the prior art. However, if production capacity should be increased, the current method and particularly the employed equipments have some drawbacks. An increase in the production capacity requires an increase in the rolling speed. The structures of current planetary rolling mills, particularly the structures of the roller heads, are ill suited to increasing the rolling speed and the rolling mill rotation speed. This is due to the centrifugal forces directed to the roller heads owing to their rotation, among others.

[0004] The object of the invention is to realize a method whereby production capacity can be increased economically. Another object of the invention is to realize an apparatus whereby the drawbacks of the prior art can be avoided and production capacity increased according to the method of the invention.

[0005] The invention is based on the observation that the working resistance of copper is diminished to a fraction after recrystallization. This enables an extremely economical further rolling, with an equipment that is remarkably more economical than in the first working step.

[0006] The invention is characterized by what is set forth in the claims.

[0007] The method according to the invention has several remarkable advantages. The division of the working process into two steps enables, among others, after the first working step, a larger wall thickness of the tube billet than in the prior art, which results in an increase in the production capacity. By means of the method and apparatus of the invention, production rates can be increased even two or three times in comparison with the prior art. The working of the tube billet—which is in the first working step recrystallized and softened mainly due to rolling—by rolling immediately after the first working step only requires a slight amount of power in the second working step. When both working steps are carried out in a protective gas chamber, harmful effects of oxidation, particularly in a copper-containing tube billet, are prevented during the working process.

[0008] In the present application, a conical roll generally means a rolling mill roll with a diameter that is at the first end of the rolling surface larger than at the second end. The true shape of the conical roll does not necessarily have to be conical or frusto-conical, but it can be varied according to the particular embodiment. Planetary rolling generally means rolling where the rolls rotate both around their own axis and around the billet to be rolled.

[0009] The invention is explained in more detail below by way of an example and with reference to the appended drawings, where

[0010]FIG. 1 is a simplified illustration of a prior art tube rolling step,

[0011]FIG. 2 is a simplified illustration of an embodiment according to the invention, and

[0012]FIG. 3 illustrates a detail of an embodiment according to the invention.

[0013]FIG. 1 illustrates a prior art solution for working a tube billet 1 by rolling. In the prior art arrangement, the tube billet 1 is planetary rolled in one working step mainly with conical roll elements 2, which will be called conical rolls in the text below. Each of the conical rolls 2 rotates around its rotary axis 3, and in addition, the rolls typically rotate essentially around the rotary axis of the planetary housing, which axis is parallel to the central axis 4 of the tube billet. During rolling, there is typically used a mandrel 5 inside the tube billet. In the drawing, the motional direction of the tube billet is indicated by the arrow 6. For the sake of clarity, the moving and drive gear of the conical rolls 2 is left out of the drawing. Some typical rolling arrangements utilizing conical rolls are disclosed for instance in the publications U.S. Pat. No. 3,735,617 and GB 2019281 A.

[0014]FIG. 2 is a simplified illustration of an embodiment according to the method of the present invention, shown in cross-section along the line A-A of FIG. 1. Accordingly, for example a continuously cast tube billet 1 is brought to a working step according to the invention. In the first working step F₁ of the method, the tube billet 1 is worked, advantageously cold worked, by rolling the conical rolls so that the temperature or the tube billet to be worked rises, mainly owing to the influence of deformation resistances, up to the recrystallization range or in the vicinity thereof, at least in the spot that is being worked. The first working step F₁ is carried out by a first rolling mill device. The first rolling mill device includes at least one, preferably several mainly conical rolls 2. In the embodiment of FIG. 2, the conical rolls 2 rotate around their axis 3 and also around the center of the planetary housing, for instance, which housing is typically located on the central axis 4 of the tube billet 1. Inside the tube billet 1, there is typically employed a mandrel 5, in which case the wall of the tube billet 1 is worked between the rolls 2 and the mandrel 5. Typically, in the first working step, the degree of working, the wall thickness of the tube billet under operation and the mass flow are chosen so that there is achieved a maximum mass flow, and that there are good conditions for recrystallization.

[0015] Essentially immediately after the first working step F₁, the tube billet is subjected to a second working step F₂, typically by rolling with a second set of conical rolls 7. At least during the first working step F₁ and the second working step F₂, and advantageously also between said working steps, the tube billet 1 is kept in non-oxidizing conditions. Said non-oxidizing conditions are created for instance by means of a protective gas space 9, where the conditions are adjusted in order to at least partly prevent the oxidation of the tube billet. The employed protective gas can typically be for example nitrogen or argon.

[0016] According to a preferred embodiment of the method according to the invention, in the second working step F₂ the wall thickness s of the tube billet 1 is diminished. Typically the wall thickness of the tube billet (1) is diminished for about 50-70% in the second working step F₂. The second working step F₂ may comprise several successive rolling steps. In a typical embodiment, in the second working step F₂ the tube billet 1 is worked by planetary skew rolling or planetary cross-rolling with conical rolls. In another embodiment, in the second working step F₂ the tube billet 1 is worked by stretch reducing. In a third embodiment, the tube billet is worked by applying sizing rolling. The second working step may comprise several successive rolling operations. Different types of working processes can also be combined in succession.

[0017] The method according to the invention provides wider possibilities for working than the prior art. In the second working step F₂, the (inner) diameter d of the tube can be maintained essentially constant. In another preferred embodiment, the tube diameter d is enlarged in the second working step F₂ (FIG. 3). The tube diameter d is enlarged by using, when necessary, a mandrel 5 inside the tube billet. In FIG. 3, the diameter of the mandrel 5 is enlarged at the second working spot conically towards the exit direction 6 of the tube billet. In a typical case, the wall thickness s of the tube billet is simultaneously diminished. In a preferred embodiment, the tube billet diameter d can also be diminished in the second working step F₂.

[0018] In the method according to the invention, the (inner) diameter d and the wall thickness s of the tube billet 1 can be adjusted to the desired measures in a way that is remarkably more flexible than those used in the prior art.

[0019] When necessary, the temperature of the tube billet 1 is adjusted, either prior to the first working step, during it, prior to the second working step or during it. Heating can be carried out for instance by using an induction coil. Naturally the billet can also be cooled in order to obtain the desired processing temperature in the tube billet.

[0020] The apparatus according to the invention for working the tube billet comprises in the first working step F₁ a rolling mill arrangement with at least one conical roll element 2. Essentially immediately after the rolling arrangement of the first working step F₁, in the proceeding direction 6 of the tube billet 1, there is arranged the rolling arrangement of the second working step F₂. The apparatus includes means for creating non-oxidizing conditions that protect the tube billet 1, said means being for example a protective gas space 9, at least at the first working step F₁ and the second working step F₂ of the rolling arrangement and advantageously also therebetween.

[0021] Typically the protective gas space 9 surrounds, at least partly, the rolling arrangement of both the first and the second working step, and also the space provided in between, at least in the vicinity of the tube billet 1. Obviously the apparatus typically also comprises means for conducting the protective gas to the protective gas space and for maintaining a sufficient protective gas content in said protective gas space.

[0022] In a typical embodiment, the diameter of the roll element of the rolling arrangement of the first working step F₁ is larger on the input side of the tube billet than on the output side (as is seen in FIG. 1). According to another embodiment, the diameter of the roll element 2 of the first rolling arrangement is larger on the tube billet output side than on the tube billet input side (according to FIG. 2). Typically the first rolling arrangement is a planetary mill with at least three conical roll elements 2 provided as the employed rolling elements.

[0023] In the embodiment of FIG. 2, at least one of the rolling arrangements of the second working step F₂ is a planetary mill.

[0024] In a preferred embodiment, the rotary axis 8 of the roll 7 of the rolling arrangement of the second working step is parallel to the longitudinal axis 4 of the tube billet 1.

[0025] Typically the rotary axis 8 of at least one roll 7 of the rolling arrangement of the second working step forms an angle with the longitudinal axis 4 of the tube billet.

[0026] In an embodiment, the rotary axis 8 of at least one roll 7 of the rolling arrangement of the second working step is essentially perpendicular to the plane that is tangential to the longitudinal axis 4 of the tube billet 1.

[0027] Thus the roll arrangement of the rolling apparatus of the second working step can consist of conical roll elements, or roll elements with rotary axes that are perpendicular to the proceeding direction of the tube billet, or of a combination of these.

[0028] The apparatus comprises at least one mandrel element 5. The shape and size of said mandrel element depends on the embodiment in question. FIG. 3 illustrates an embodiment where the (inner) diameter d of the tube billet 1 is enlarged. At the same time, the wall thickness s of the tube billet 1 is diminished. The diameter of the mandrel 5 is enlarged conically at the working spot towards the output direction 6 of the tube billet 1.

[0029] The invention is mainly suited to the manufacturing of tubes made of a non-ferrous material. In particular, the invention is designed to the manufacturing of copper or copper alloy tubes. 

1. A method for manufacturing a tube made of a non-ferrous material, particularly made of mainly copper, by means of rolling, in which method in the first working step (F₁), a tube billet (1) is worked by rolling with conical rolls (2), so that mainly owing to deformation resistance, the temperature of the billet under operation rises up to the recrystallization range, at least on the working spot, characterized in that essentially immediately after the first working step (F₁), the tube billet (1) is subjected to at least one second working step (F₂) by rolling with a second set of conical rolls (7), so that at least during the first working step (F₁) and the second working step (F₂), the tube billet (1) is kept in non-oxidizing conditions.
 2. A method according to claim 1, characterized in that the tube billet is kept in non-oxidizing conditions also between the different working steps.
 3. A method according to claim 1 or 2, characterized in that the non-oxidizing conditions are provided by means of a protective gas chamber (9) which contains protective gas.
 4. A method according to any of the claims 1-3, characterized in that in the first working step (F₁) the tube billet (1) is cold worked by rolling with conical rolls (2).
 5. A method according to any of the claims 1-4, characterized in that the wall thickness (s) of the tube billet (1) is diminished in the second working step (F₂).
 6. A method according to any of the claims 1-5, characterized in that wall thickness (s) of the tube billet (1) is diminished for about 50-70% in the second working step (F₂).
 7. A method according to any of the claims 1-6, characterized in that the diameter (d) of the tube billet (1) is kept essentially constant in the second working step (F₂).
 8. A method according to any of the claims 1-7, characterized in that the diameter (d) of the tube billet (1) is diminished in the second working step (F₂).
 9. A method according to any of the claims 1-8, characterized in that the diameter (d) of the tube billet (1) is enlarged in the second working step (F₂).
 10. A method according to any of the claims 1-9, characterized in that the second working step (F₂) includes several successive rolling operations.
 11. A method according to any of the claims 1-10, characterized in that in the second working step (F₂), the tube billet (1) is worked by planetary rolling.
 12. A method according to any of the claims 1-11, characterized in that in the first working step (F₁), the tube billet (1) is worked by planetary rolling.
 13. A method according to any of the claims 1-12, characterized in that the tube billet (1) is a continuously cast billet.
 14. A method according to any of the claims 1-13, characterized in that the temperature of the tube billet (1) is adjusted when necessary.
 15. An apparatus for working a tube billet (1) by rolling, said apparatus comprising, in order to perform the first working step (F₁), a planetary rolling arrangement including at least one conical roll element (2), characterized in that essentially immediately after the first rolling arrangement, in the proceeding direction (6) of the tube billet (1), there is arranged a second rolling arrangement in order to perform at least one second working step (F₂), and that the arrangement comprises means for creating non-oxidizing conditions at least at the first and second rolling arrangement and advantageously between them, too.
 16. An apparatus according to claim 15, characterized in that the means for creating non-oxidizing conditions comprise a protective gas chamber (9) for protecting the tube billet (1).
 17. An apparatus according to claim 15 or 16, characterized in that the protective gas chamber (9) surrounds the first and second rolling arrangement and the space provided therebetween at least in the vicinity of the tube billet (1).
 18. An apparatus according to any of the claims 15-17, characterized in that the diameter of the conical roll element (2) of the first rolling arrangement is, in the proceeding direction (6) of the tube billet, larger on the input side than on the output side (FIG. 1).
 19. An apparatus according to any of the claims 15-17, characterized in that the diameter of the conical roll element (2) of the first rolling arrangement is, in the proceeding direction (6) of the tube billet, larger on the output side than on the tube billet input side (FIG. 2).
 20. An apparatus according to any of the claims 15-19, characterized in that the first rolling arrangement is a planetary mill, where at least three conical roll elements (2) are provided as rolling elements.
 21. An apparatus according to any of the claims 15-20, characterized in that at least one other rolling arrangement is a planetary mill.
 22. An apparatus according to any of the claims 15-21, characterized in that the rotary axis (8) of at least one roll (7) of the second rolling arrangement forms an angle with the longitudinal axis (4) of the tube billet (1).
 23. An apparatus according to any of the claims 15-22, characterized in that the rotary axis (8) of the roll (7) of the second rolling arrangement is parallel with the longitudinal axis (4) of the tube billet (1).
 24. An apparatus according to any of the claims 15-23, characterized in that the apparatus comprises at least one mandrel element (5). 